Various types of surgical procedures are currently performed to investigate, diagnose, and treat cardiovascular diseases. Using current techniques, many of these procedures require a gross thoracotomy, usually in the form of a median sternotomy, to gain access to the patient's thoracic cavity. A saw is used to cut the sternum longitudinally thereby allowing two opposing halves of the anterior or ventral portion of the rib cage to be spread apart. A large opening in the thoracic cavity is created through which the surgical team may directly visualize and operate upon the heart and other thoracic contents.
Surgical intervention in the heart generally requires isolation of the heart and coronary blood vessels from the remainder of the arterial system and arrest of cardiac function. The heart is usually isolated from the arterial system by introducing an external aortic cross-clamp through a stemotomy and applying the clamp to the aorta between the brachiocephalic artery and the coronary ostia. Cardioplegic fluid is then injected into the coronary arteries, either directly into the coronary ostia or through a puncture in the aortic root, to arrest cardiac function. In some cases, cardioplegic fluid is injected into the coronary sinus for retrograde perfusion of the myocardium. The patient is then placed on cardiopulmonary bypass to maintain peripheral circulation of oxygenated blood. Another method of arresting the patient's heart is disclosed in U.S. Pat. No. 5,433,700, which is assigned to the assignee of the present application and is herein incorporated by reference. U.S. Pat. No. 5,433,700 describes an endovascular catheter system for establishing arrest of cardiac function. The endovascular catheter system does not require a gross thoracotomy and facilitates less invasive methods of performing cardiopulmonary procedures.
Once the patient is placed on cardiopulmonary bypass, various surgical techniques may be used to repair a diseased or damaged valve, including annuloplasty (contracting the valve annulus), quadrangular resection (narrowing the valve leaflets), commissurotomy (cutting the valve commissures to separate the valve leaflets), shortening mitral or tricuspid valve chordae tendonae, reattachment of severed mitral or tricuspid valve chordae tendonae or papillary muscle tissue, and decalcification of valve and annulus tissue. Alternatively, the valve may be replaced, by excising the valve leaflets of the natural valve and securing a replacement valve in the valve position usually by suturing the replacement valve to the natural valve annulus. Various types of replacement valves are in current use, including mechanical and biological prostheses, homografts, and allografts, as described in Bodnar and Frater, Replacement Cardiac Valves 1-357 (1991), which is incorporated herein by reference. A comprehensive discussion of heart valve diseases and the surgical treatment thereof is found in Kirklin and Barratt-Boyes, Cardiac Surgery , pp. 323-459 (1986), the complete disclosure of which is incorporated herein by reference.
The mitral valve, located between the left atrium and left ventricle of the heart, is most easily reached through the wall of the left atrium, which normally resides on the posterior side of the heart, opposite the side of the heart that is exposed by a median sternotomy. Therefore, in order to access the mitral valve via a sternotomy, the heart is rotated to bring the left atrium into an anterior position accessible through the sternotomy. An opening, or atriotomy, is then made in the right side of the left atrium, anterior to the right pulmonary veins. The atriotomy is retracted by means of sutures or a retraction device, exposing the mitral valve directly posterior to the atriotomy. One of the aforementioned techniques may then be used to repair or replace the valve.
An alternative technique for mitral valve access may be used when a median sternotomy and/or rotational manipulation of the heart are undesirable. In this technique, a large incision is made in the right lateral side of the chest, usually in the region of the fifth intercostal space. One or more ribs may be removed from the patient, and other ribs near the incision are retracted outward to create a large opening into the thoracic cavity. The left atrium is then exposed on the posterior side of the heart, and an atriotomy is formed in the wall of the left atrium, through which the mitral valve may be accessed for repair or replacement.
Using such open-chest techniques, the large opening provided by a median sternotomy or right thoracotomy enables the surgeon to see the mitral valve directly through the left atriotomy, and to position his or her hands within the thoracic cavity in close proximity to the exterior of the heart for manipulation of surgical instruments, removal of excised tissue, and/or introduction of a replacement valve through the atriotomy for attachment within the heart. However, these invasive, open-chest procedures produce a high degree of trauma, a significant risk of complications, an extended hospital stay, and a painful recovery period for the patient. Moreover, while heart valve surgery produces beneficial results for many patients, numerous others who might benefit from such surgery are unable or unwilling to undergo the trauma and risks of current techniques.
A problem which occurs in conventional open-heart procedures is that air enters the heart during the procedure and must be removed from the heart after completing the procedure. Air which remains in the circulatory system after the heart is closed may produce air emboli which could travel to the brain and cause a stroke or death. Conventional de-airing techniques include mechanical manipulations and venting of the heart to remove air trapped in the heart. U.S. Pat. No. 5,370,631, for example, discloses an apparatus for de-airing the heart which includes a slotted-needle and a resilient bulb.
Carbon dioxide has been used to displace air in the patient's thoracic cavity to help prevent air emboli. In animal studies, carbon dioxide has been shown to be as much as twelve times more soluble in blood than air. Thus, displacing air with carbon dioxide may be beneficial in reducing the harmful effects of gas emboli.
In open-heart procedures, carbon dioxide has been introduced into the thoracic cavity through the median sternotomy. Since the patient's chest is open, the carbon dioxide in the chest cavity readily disperses out of the chest and, therefore, carbon dioxide must be continuously or periodically replaced, Ng and Rosen, "Carbon Dioxide in the prevention of air embolism during open-heart surgery", Thorax 23:194-196 (1968).
Thus, a problem with previous use of carbon dioxide in open heart procedures is that air is free to enter the open chest cavity and high carbon dioxide concentrations cannot be maintained in the chest cavity for extended periods of time without requiring continuous or periodic injection of carbon dioxide.